"Magic Mushrooms" is ambiguous, so yeah, anything around it is a myth. When it comes to Cubensis, it's commonly found in the places you mention. I think the "myth" you mention, is about Liberty Cap (Psilocybe semilanceata) which you can find basically anywhere in the northern hemisphere, very common all over Europe. While Liberty Caps don't directly grow in the dung, they do love cow and sheep pastures and is more commonly found there. If you're an adult, never experienced psychedelic drugs before but would like to, and live in Europe, try looking for Liberty Caps in a nearby pasture once temperatures went down to between 0-10 degrees. I'm sure you'll find some.
Not sure that's true, some adventurers get old, some don't. Boldness has nothing to do with it. Shulgin, Leary and others reach high age, still bold people.
You say cow dung as if you think that rich, picky concoction of nutrients (vegetation processed by a ruminant animal and then shat onto the ground) is an example of it being the opposite of picky.
Magic mushrooms, in particular, love cow dung as a growing medium.
That being said, fungus tends to be nigh indestructible. The guy boiled it before injection which really speaks to just how hardy those spores can be.